1. Field of the Invention
The present invention relates to tuning systems for selectively tunable heterodyne receivers.
2. Description of the Prior Art
Tunable super heterodyne receivers are known and an example is disclosed in German Patent No. 1,076,265. The receiver in this patent includes a number of frequency conversion stages with the second injection oscillator having its output signal converted to a significantly lower frequency position by an additional interpolation oscillator by means including a frequency stabilized auxiliary voltage. The second oscillator has its frequency adjusted to the value of the adjusted frequency of the interpolation oscillator by use of an automatic frequency control system. Thus, if the output frequency of the second oscillator is reduced by at least a factor of 10.sup.2 and if this frequency band tuning range of the interpolation oscillator is selected to be correspondingly low, the second oscillator may be adjusted with the use of the interpolation oscillator to a precise frequency. However, in spite of the continuous frequency variation, the individual frequencies of the second oscillator are as stable as if they were controlled by a crystal oscillator. Thus, this receiver has a very high tuning accuracy and stability for the heterodyne receiver with respect to each random tuning frequency within a given frequency range. The indication of the coarse tuning takes place on a coarse tuning scale associated with the first injection oscillator and the indication of the fine tuning occurs on the tuning scale associated with the interpolation oscillator. However, particularly due to the stabilization measures of such prior art systems, the cost of the receiver of the prior art is very high and the system is relatively complicated.